The effects of morphine and naloxone on serum levels of luteinizing hormone in intact and ovariectomized pony mares during the breeding and nonbreeding seasons

Graduation date: 1984Intact and ovariectomized (OVX) pony mares were randomly assigned to treatments of morphine (M), naloxone (N), or a saline control (C) to determine if endogenous opioid peptides (EOP) play a role in equine seasonal breeding. Morphine was expected to decrease luteinizing hormone (LH), whereas N (an opiate antagonist) should in crease it. These opiate analogs were given to normally cycling mares on the second day of estrus, and to winter anestrous mares having no follicular activity (follicles less than 5 mm diameter). All treatment groups (n=3) received a single injection (1 mg/kg body weight, iv) on zero hour, except the winter N treatments which had three hourly doses (1 mg/kg body weight, iv). Serial blood samples taken on treatment day showed no difference (P>.05) in LH patterns in the winter (M, 1.4±.1; N, 1.7±.1; C, 1.7±.1 ng/ml; x̄±SE). In the summer, LH patterns (M, 9.8±.2; N, 20.1±.9; C, 16.5±.5) were different (P<.01) between M and N treatments, but not when compared to controls. However, this difference seemed to be due to the 24 h sample of one N treated pony, which had high LH levels. Days in estrus were unchanged in all cycling ponies (M, 7.0±0.0; N, 8.3±2.3; C, 7.3±.9 d). Three ponies that received daily injections of M throughout estrus also had similar lengths of estrus (9.0±1.0). Two OVX ponies were treated three times each on a switchback type of experiment in the summer and the winter. Again, there was no difference in LH except that due to season (summer, 33.6±1.0; winter, 1.4±.l ng/ml; P<.05). These opiate analogs at this treatment dose indicated that EOP may not play a major role in equine reproduction as related to LH